The present invention relates, in general, to a method of treating hydrocarbons. More particularly, the present invention pertains to a liquid-phase adsorption process, which removes heteroatom compounds such as phenols, naphthenic acids, pyridines, pyrroles, thiophenes, quinolines or carbazoles from a liquid hydrocarbon stream, and concentrates them into heteroatom compound products.
Hydrocarbons include compounds that contain heteroatoms such as nitrogen, sulfur and/or oxygen. Such heteroatom compounds in hydrocarbons are exemplified by phenols, naphthenic acids, pyridines, pyrroles, thiophenes, quinolines and carbazoles, of which polarities are higher than those of hydrocarbons. For the sake of convenience, heteroatom compounds in hydrocarbons are abbreviated, hereinafter to xe2x80x9cHCHxe2x80x9d.
As the distillation cut point of petroleum crude increases, the HCH content within the distillate increases. Physical properties and compositions of HCH vary with different petroleum crudes, however, HCH are generally considered as undesirable constituents: naphthenic acids corrode metals and poison catalysts; phenols from a FCC (Fluid Catalytic Cracking) process degrade fuel quality; and basic organic nitrogen compounds are known to interfere with hydrodesulphurization reactions.
Adsorption has long been used to remove or concentrate polar compounds such as HCH. In commercialization of an adsorption process, however, development of an effective regeneration method is essential, since an adsorbent has a finite adsorption capacity. In general, regeneration can be carried out by means of thermal energy, chemical reaction, inert gas or solvents. The present invention uses a polar solvent, and an oxygen-containing polar solvent such as an alcohol, a ketone, or an ether, can be used to regenerate the adsorbent. Applied to the adsorbent after adsorption is completed, the polar solvent purges the adsorbate and returns a regeneration effluent, which is an adsorbate-polar solvent mix. Adsorbate is readily separable from the regeneration effluent subject to distillation, however, the problem is that the adsorbate includes not only HCH, but also hydrocarbons. In fact, the hydrocarbon portion in the adsorbate is much larger than the HCH portion in the adsorbate, and this is referred to as coadsorption phenomenon. HCH quantities in the adsorbate are merely 10% or less, while hydrocarbons take up the rest of the adsorbate when kerosene or LGO (Light Gas Oil) are adsorption-treated. Under coadsorption conditions, therefore, process yield, i.e. hydrocarbon yield, is reduced. To resolve this problem, a special step, which selectively displaces coadsorbed hydrocarbons from the adsorbate prior to regeneration, is introduced. This step is referred to as xe2x80x9ccopurgingxe2x80x9d. Typical copurging methods, which are dependant upon hydrocarbons, are as follows:
In case the feedstock has a low boiling point, e.g., the feedstock is naphtha:
Naphtha with a boiling point range of 80 to 130xc2x0 C. is fed to an adsorber in the liquid phase in an upward direction, and an adsorption effluent, i.e., a naphtha-solvent mix, is returned. The adsorption effluent is then separated from the solvent in a fractionator. Copurging is carried out by injecting a vaporized naphtha stream into the adsorber in a downward direction to displace coadsorbed naphtha from the adsorber. Finally, solvent flushes the adsorber in the downward direction, purging HCH from the adsorber. Regeneration effluent, which is an HCH-solvent mix, is separated in the other fractionator, while the copurging effluent is adsorbed again in the beginning of a subsequent adsorption.
In case the feedstock has a high boiling point, e.g., the feedstock is LGO:
LGO requires a lot of energy to be fully evaporated. Copurging is therefore conducted with a non-polar solvent as depicted in FIG. 1. LGO, of boiling point range 225xc2x0 C.xcx9c365xc2x0 C., is fed to an adsorber in an upward direction. After adsorption is completed, copurging is carried out by applying a non-polar solvent such as hexane to the adsorber, in a downward direction, to selectively desorb coadsorbed LGO. Finally, in the downward direction, a polar solvent such as MTBE (Methyl-Tertiary-Butyl-Ether) is injected to the adsorber to purge remaining adsorbate. This process returns a small amount of a highly concentrated HCH product, but nevertheless, requires 4 fractionators to recover the two different solvents. Operating costs are also high because separation of the two solvents is costly, and copurging effluent is adsorbed again due to its high HCH content.
It is an object of the present invention to provide a process for removing and concentrating heteroatom compounds from a liquid hydrocarbon stream, in which adsorption, copurging and regeneration are conducted in an adsorber that is charged with silica gel and maintained in a fully wetted condition, comprising the following steps:
i) feeding the liquid hydrocarbon stream to the adsorber, and returning a first effluent, which is a polar solvent-rich mix, to a second fractionator before sending a second effluent, which is a polar solvent and adsorption treated hydrocarbon mix, to a first fractionator;
ii) injecting a fraction of heteroatom compounds that are extracted in the previous operation to the adsorber, and returning a third effluent, which is a polar solvent and adsorption treated hydrocarbon mix, to a first fractionator before sending a fourth effluent, which is an adsorption treated hydrocarbon and heteroatom compounds mix, to a feed drum for re-adsorption; and
iii) applying the polar solvent to the adsorber, and returning a fifth effluent, which is a heteroatom compound rich mix; to a heteroatom compound drum before sending a sixth effluent, which is a heteroatom compounds and polar solvent mix, to the second fractionator.
In the present invention, the liquid hydrocarbon comprises middle distillates with a boiling range of 100 to 400xc2x0 C. and the heteroatom compounds are nitrogen atom containing compounds and oxygen atom containing compounds. The first fractionator separates adsorption-treated hydrocarbons from the polar solvent, which are reused in regeneration. The second fractionator separates heteroatom compounds, of which a fraction is taken out as a HCH product and the residue is used in copurging, from the polar solvent, which is reused in regeneration.
As mentioned above, to remove and concentrate HCH, a liquid phase adsorption process has been developed, which comprises steps of adsorption, copurging and regeneration. In an effort to attain commercial feasibility, an adsorbent, a polar solvent, and a copurging method are carefully selected. Silica gel is identified as a suitable adsorbent for its regenerability as well as ability to maintain its adsorption effectiveness; MTBE (Methyl-Tertiary-Butyl-Ether), ETBE (Ethyl-Tertiary-Butyl-Ether) or TAME (Tertiary-Amyl-Methyl-Ether) is selected for its low latent heat and compatibility with the adsorbent; and the copurging method is developed to effectively displace coadsorbed hydrocarbons
The operation sequence of the process is illustrated in FIG. 3 and FIG. 4: in xe2x80x9cStep 1xe2x80x9d, a predetermined amount of hydrocarbon is fed to an adsorber, which is maintained in the vicinity of 50xc2x0 C., in an upward or downward direction; in xe2x80x9cStep 2xe2x80x9d, copurging is carried out by injecting HCH, which have been extracted and set aside from the previous regeneration, to the adsorber, displacing coadsorbed hydrocarbons from adsorbate, in an upward or downward direction; in xe2x80x9cStep 3xe2x80x9d, a polar solvent such as MTBE is applied to the adsorber, purging remaining adsorbatexe2x80x9d; and in xe2x80x9cStep 4xe2x80x9d, a fraction of HCH, which is obtained in xe2x80x9cStep 3xe2x80x9d, is taken out as an HCH product while the rest is reserved for the next copurging.
A continuous adsorption unit, shown as FIG. 6, is built according to the present invention. The process repeats the aforementioned steps until reaching an equilibrium. As a result, the present invention is feasible. As expected, 60% or more of HCH are removed from the hydrocarbon feedstock while as little as 2.0% of an HCH product is generated.